The automotive, consumer appliance, consumer electronics, and other industries require adhesives that have a combination of properties such as long durability, resistance to continuous exposure to high temperatures, and good peel adhesion to a wide range of substrates such as metals, plastics, paints and the like. While rubber/resin based adhesives show excellent peel adhesion to many substrates, they perform poorly at elevated temperatures. Acrylic based adhesives, in contrast, either have outstanding peel adhesion and poor elevated temperature performance or low peel adhesion and outstanding elevated temperature performance. What is desired is a durable adhesive with good peel adhesion to low surface energy (LSE) substrates, high shear adhesion failure temperature, good stress relieving properties, and excellent converting properties.
It is known that peel adhesion to various substrates is generally improved if acrylic pressure-sensitive adhesives are compounded with low molecular weight tackifying resins. For example, Japanese Patent 8569180 (assigned to Nitto) teaches the use of terpene phenolic resin as a tackifier in photocurable pressure-sensitive adhesives. The use of other types of resins, including rosin resin, is described in the Japanese Journal “Setchaku” (Vol. 23, p. 489-504, 1984). The use of cyclohexene resin is descried in DE 323122 (1983).
U.S. Pat. No. 4,418,120 to Kealy et al., discloses a cross-linked tackified polymer based on isooctylacrylate and 3 to 7% by weight acrylic acid. The polymer is stated to have an inherent viscosity of 0.75 to 1.5 dl/g prior to cure. The cured product was stated to have shear value at 70° C., of at least 5,000 minutes. Incorporating low molecular weight resins into the adhesive formulation typically results in adhesives which have poor high temperature shear adhesion.
The durability, adhesiveness, and other properties of pressure sensitive adhesives are also affected by both the choice of monomers which comprise the copolymer, and the type of curing employed. Japanese patent No. 84-18774 discloses a radiation curable pressure-sensitive composition containing a terpene-phenolic resin and glycidyl methacrylate monomer. The cure is through carbon-carbon double bonds and is activated by radiation.
U.S. Pat. No. 6,893,718B2 to K. Melancon et al., discloses PSA composition containing a bisamide crosslinking agent which provide high temperature resistance and adhesion, but these types of PSAs do not adhere well to low surface energy substrates such as polypropylene substrates. Similarly, U.S. Pat. No. 4,456,741 to Ames discloses a hot melt acrylic adhesive containing tackifiers having good peel adhesion and permanent tack, but low shear adhesion.
U.S. Pat. No. 4,726,982 to Traynor et al., discloses a tackified acrylic based pressure sensitive composition containing N-vinyl-2-pyrrolidone, and described as being useful in adhering to high solids automotive paints.
U.S. Pat. No. 7,109,266 to Isiguro et al., discloses acrylic pressure sensitive adhesive composition containing an acrylic polymer and a tackifier obtained by acid modification of a tackifier resin inherently free of an acid component which makes PSA composition having high initial adhesive strength.
U.S. Pat. No. 5,639,811 to Plamthottam et al., teaches tackified pressure sensitive adhesives and tapes comprising a copolymer based on an acrylic backbone, with a glycidyl monomer, unsaturated carboxylic acid monomer and preferably a vinyl lactam monomer, and a tackifier, said adhesives being dual curable and exhibiting outstanding peel adhesion and high temperature shear performance. However, molecular weight was not sufficiently high to pass the extremely high temperature shear requirements.
WO 2008/116033 teaches pressure sensitive adhesive formed from a blend comprising: a) an acrylate polymer having a Tg less than 20° C. and a molecular weight greater than 20,000, b) an oligomer having at least one crosslinkable functional group, wherein the oligomer is liquid at room temperature, and c) at least one crosslinking agent, wherein at least one of a) and b) is crosslinked in the presence of the other.
US patent 2005/0061435 A1 to Everaerts et al., discloses latent over-tackified adhesive that can be activated with a plasticizer upon demand to form a pressure sensitive adhesive. This formulation utilizes aziridine crosslinker but does not enhance the high temperature shear during the product use.
U.S. Pat. No. 5,874,143 to Peloquin et al., discloses incorporation of a plasticizer material into a tackified acrylic PSA. The resulting PSA is shown to provide improved bonding of the marking films and labels to low energy surfaces such as high density polyethylene plastic.
Despite all the attempts, a PSA having combination of high peel adhesion to low surface energy substrates, high temperature shear, excellent stress relieving properties, and excellent converting properties is not available. For many industrial tape bonding applications with new materials, the PSA are still required to have a combination of properties such as high cohesive strengths, adhesion to low surface energy substrates, and stress relieving properties.